1) Field of Disclosure
The present disclosure is related to the field of building materials. This disclosure has particular applicability to coatings for building materials having thermochromic properties.
2) Description of Related Art
Mankind has always sought methods to improve his living conditions. The development of protective shelters provided needed safety from harsh environments. Cooling and heating systems powered by fossil fuels greatly increased the comfort of these shelters. However, there is a desire to reduce the amount of energy expended for cooling and heating for a multitude of reasons. For example, reasons include lowering the financial cost to the building owner and reducing the environmental impact from the use of fossil fuels. These reasons provide motivation to develop non-energy using means to mediate the environment of interiors.
One way of reducing the amount of energy necessary to heat and cool a structure is to alter the color of the structure's exterior. As is well known, light colors, such as white, reflect sunlight, which in turn reduces the degree to which sunlight heats up a light-colored surface. Conversely, darker colors absorb sunlight, which cause a dark surface to heat up under exposure to sunlight. Previously, however, application of this principle to buildings and other structures has not been deemed a reasonable solution, as the cost and effort necessary to periodically alter the color of structures, such as by painting or covering, was prohibitive.
Thermochromic compositions change color in response to temperature fluctuations. Conventional reversible thermochromic compositions exhibit reversible thermochromic properties such that they begin to become color-extinguished in the course of temperature rise caused in a color-developed state, present a completely color-extinguished state at a specific temperature or above, and begin to develop a color in the course of subsequent temperature drop and return to the color-developed state.
The thermal or “UVI” radiation range is the range of frequencies in the Ultraviolet, Visible and Infrared ranges which produce heat in objects, and which absorb those frequencies. The relative transparency/opacity of a thermochromic material is dynamic with respect to the trigger temperature or trigger temperature range of the specific thermochromic material. For example, a thermochromic material having a trigger temperature of precisely 72 degrees Fahrenheit would be opaque to light and thus would absorb thermal radiation in the UVI range at temperatures below 72 degrees Fahrenheit and transparent, or non-absorptive, to UVI light at temperatures above 72 degrees Fahrenheit.
Another desired property of shelters is to provide long term protection without the need for extensive repair. Rain, heat and wind cause damage to exterior surfaces and as such, a means to prevent the deterioration of these surfaces is an ever present goal in the field of structure design. One form of protection is the use of polyurethane coatings to protect exposed surfaces from harmful elements. For example, U.S. Pat. No. 4,710,560 is directed toward a hydrophobic, crosslinked polyurethane for use as a corrosion inhibiting coating on exposure to moisture at atmospheric conditions.
However, although the coating of a polyurethane resin can show various kinds of properties depending upon the composition thereof, an important defect of the polyurethane resin is the discoloration and the deterioration such as hydrolysis, when exposed to the outdoors over a long period of time. Therefore, polyurethane resin coatings can be unsuitable for uses in which good durability is required.
It is also known that thermochromic materials can be added to thermoplastics, polyvinyl chloride (PVC) or other resins and molded into any shape or design or made into sheets. The use of thermochromic materials in the coatings of structures has been explored. For example, Khaldi (U.S. Pat. No. 6,500,555) discloses thermochromic laminates, which predictably vary their ability to absorb or reflect electromagnetic radiation. However, without the proper medium for which the thermochrome is carried, the thermochrome and the coating are quickly deteriorated upon exposure to the elements.
Smith also teaches, in US Application 2008/0209825, a color changing system for structures that uses polyurethane laminates in a layered structure. However, as indicated above, problems occur with the use of polyurethanes alone.